The present invention relates to a color image forming apparatus such as a color copying machine, color printer, or the like for reproducing on an image output medium a color image corresponding to an original color image formed on an image input medium. The present invention further relates to a color image forming method. More particularly, the present invention relates to a color image forming apparatus and method thereof capable of reproducing a color image with sufficient reproducibility, even in the case where a color-reproducible range (gamut) of the image input medium is inconsistent with that of the image output medium.
There has been known a color image forming apparatus of a type in which an original color image formed on an image input medium is read out by an image reader to obtain an original image data for each of picture elements of the original color image. The original image data consists of digital signals of separated color components (such as red, green and blue components) representing density or reflective coefficient of the picture element color. The original image data is color-proofed or transformed into a control signal data. The control signal data will control a recording portion to reproduce on the image output medium a color the same as the picture element color on the image input medium. The control signal data consists of cyan, magenta and yellow color component control signals. The recording portion performs a recording operation with cyan, magenta and yellow color materials in accordance with the control signal data, to thereby reproduce a color image on an output medium corresponding to the original image on the input medium.
In the above-described image forming apparatus, the image formed or reproduced on the image input medium is determined with the original image data represented by an RGB color system. Then, the RGB system image data is converted into another data represented by a CMY color system through the color-proofing operation so that the CMY color materials may be recorded on the image output medium in accordance with the CMY color system.
The color-proofing operation is performed, since a color system used for determining or representing the image on the image input medium is different from that for reproducing or representing the image on the image output medium. In other words, the color-proofing operation is performed for transforming a color (first color) on the image input medium which is represented by one color system (first system) into another color (second color) represented by another color system (second system) so that the second color reproduced by the second system may become the same color as the first color formed by the first system.
The image input and output media include several types of media such as a photographic film, television, copy sheet, and coated or non-coated sheet. Color-reproducible ranges (gamuts) of the image input and output media are different from one another, in accordance with their manners of forming or reproducing colors thereon. There frequently occurs, therefore, such a case that the image input and output media for a single color image forming apparatus have their gamuts different from each other.
In the case where the image input gamut is thus inconsistent with the image output gamut, colors reproduced on the output medium are made different from colors of the original image which has been formed on the input medium, as a result of which color-reproducibility of the image forming apparatus is degraded. Furthermore, in the case where the gamut of the image input medium is larger than that of the image output medium, a color of the original image located inside of the image input gamut but outside of the image output gamut may not be reproduced on the image output medium.
In order to correct such a color-inconsistency and improve the color-reproducibility of the image forming apparatus, a color-compression processing is generally conducted before the above-described color-proofing. In other words, the color of the original image data obtained from the image input medium is subjected to a color-compression processing before subjecting to the color-proofing processing.
The color-compression technique (or a data-compression transform technique) is disclosed in Japanese Unexamined Patent Application Publication No.Sho 63-254889. In the color-compression technique, a color of the original image which is located inside of the image input gamut but outside of the image output gamut is compressed in a direction toward an achromatic color, thereby to be transformed into a color as located inside of the image output gamut. In other words, the color-compression process is conducted for shifting (color-compressing) a color within the image input gamut to a color within the image output gamut in a color space of a color system representing the original image data.
Since the thus color-compressed original image data is subjected to the color-proofing operation to be converted into a control signal data, the control signal data may control the recording portion to record, on the image output medium, such a color image having the same colors as the original image on the image input medium.
The achromatic color used in the color-compression processing is defined as a color having a lightness, but no hue and chromaticness (saturation). White, gray and black colors belong to the achromatic color, and are decreased in lightness in this order. In the following description, a color axis on which the achromatic colors having various lightnesses (white, gray and black) are located is referred to as an "achromatic color axis".